PURPOSE: To prevent the impurities such as iron from being mixed into a ground raw material by supplying ceramics and a firing adjuvant at a constant ratio as the raw material to be ground and constituting both the grinding media and the lining material of a grinding device of the same material as ceramics.

CONSTITUTION: The material of both a liner 6a of a rotary pan 6 and a liner 7a of an outer circumferential ring 7 is constituted of the same ceramics as a raw material. The ceramics balls made of the same material as ceramics for the raw material are housed in a grinding chamber surrounded by the rotary pan 6 and the outer circumferential ring 7. The material to be ground is introduced and also the rotary pan 6 is rotated via a driving wheel 2 to grind the raw material. The mixing ratio of silicon nitride and a firing adjuvant (alumina, yttrium oxide) is set to (90-98) : (10-2) suitable for molding. This mixture is continuously supplied to a grinding device 500 by three feeders 101, 201, 301 and a product being the mixed powder is continuously obtained from a bag collector. Mixing of the impurities resulting from wear accompanied with grinding is not thoroughly caused because the material of the balls and the liners is same as ceramics of the main raw material.